The objective of this project is to understand the mechanism of action of the hypolipidemic drug, clofibrate, in terms of its biochemical effects on fatty acid oxidation by rat liver. Preliminary experiments indicate marked increases in the rate of fatty acid oxidation upon clofibrate treatment. We will measure the activities of the enzymes of fatty acid metabolism in the livers of normal and clofibrate-treated rats. We will also measure the subcellular localization of these enzymes, in view of preliminary evidence that at least some of the enzyme activities increased by clofibrate are located in peroxisomes, while others are in the mitochondria. Peroxisomes and mitochondria will be purified by differential and equilibrium density centrifugation from the livers of normal rats and rats treated with clofibrate. The activities of each of the enzymes of fatty acid oxidation will be assayed in the purified particles. The oxidation of radioactive fatty acids will also be studied and the products characterized be means of silica gel columns. The proteins of the peroxisomes will be fractionated and the lipid-metabolizing enzymes analyzed in terms of their substrate specificities, and enzymatic, physical, and optical properties. The characteristics of the peroxisomal enzymes will be compared with those of their mitochondrial counterparts. These studies should provide a biochemical explanation of the mechanism of action of clofibrate in reducing serum lipids.